I guess if Sigma warranty card does not state receipt is needed, then there's no need for it then. But quite often, they have a clause stating that they have the right to reject your warranty card and/or repairing your item as they see fit. Do read carefully.
Nowadays, most camera manufacturers have already wised up to this "unfilled warranty" thing that many people have been pulling off for years. Last time Konica-Minolta don't need receipt, but when Sony took over, they really implemented all kinds of safeguards to ensure this kind of thing doesn't happen.
So yes, in probably 90% of the cases, you will need a receipt to accompay the w'ty card. (But in the case of Nikon, if you lose your receipt, don't worry, they can track down where and when you bought your camera from - personal experience.)